1. Field of the Disclosure
Embodiments disclosed herein relate generally to marine riser buoyancy modules. In particular, embodiments disclosed herein relate to marine riser buoyancy modules configured to reduce vortex-induced vibration.
2. Background Art
Offshore oil and natural gas drilling and production, particularly in deep water, relies on substantially vertical conduits called “marine risers” to convey fluids and slurries between the seabed and the surface, including but not limited-to, drilling risers, production risers, export risers, steel catenary risers (“SCRs”), and flexible composite flowlines.
Some marine risers, such as SCRs, may include a single conduit, while other risers, such as drilling risers, may include a larger-diameter main conduit with a plurality of attached, smaller diameter auxiliary lines, including but not limited-to, choke and kill lines, “boost” lines, and hydraulic supply and control lines. In some cases, electrical or fiber optic control umbilicals may also be attached to the main conduit of the marine riser.
Typically a marine riser may be at least partially supported by floatation of one form or another, including for example evacuated buoyancy “cans” or buoyancy modules made from, for example, syntactic foam material. Buoyancy modules may be arranged circumferentially around the main conduit of a marine riser. Marine drilling risers, for example, typically have syntactic foam buoyancy modules, each including two “clamshell” longitudinal half-cylinder buoyancy elements that are clamped around the main conduit, and which have molded-in grooves, recesses and holes to accommodate attachment hardware and auxiliary lines.
Other types of marine risers may have evacuated buoyancy “cans” which may be generally toroidal (i.e., doughnut-shaped) and slipped over the main conduit, or may have evacuated buoyancy “cans” of other forms (e.g., closed-end cylinders) arranged in a circumferential array around the main riser conduit. Sometimes, buoyancy cans may be connected to the surface by piping so that water may be evacuated from the cans by high-pressure gas (such as compressed air or nitrogen) or by a buoyant slurry comprising, for example, glass microspheres.
If marine risers are installed in an area having significant currents, especially, for example, in or near the Loop Current in the Gulf of Mexico, or near the mouth of the Amazon River in Brazil, the current may induce a significant lateral drag force (usually simplified to “drag”) on the marine riser, and in some cases may also induce the marine riser to vibrate or “strum,” typically in a plane perpendicular to the current direction, in what is called Vortex-Induced Vibration, or “VIV.” In some cases, VIV may be reduced by changing the natural frequency of the riser string, as by increasing the tension on the string, but this has the side-effect of further loading the marine structure from which the marine riser is suspended (e.g., a drilling vessel such as a drillship or semisubmersible, or a tension-leg platform, or a floating production vessel).
Drag on the riser string may increase the bending loads on the riser joints and attached equipment, increase the load on riser tensioning equipment, and increase loads on mooring equipment and/or increase energy costs for dynamic positioning. In some cases, drag may make it difficult or impossible to run or retrieve a riser, especially if the weather is unsettled. Additionally, VIV may reduce the fatigue life of a riser string.
Furthermore, in deep water, marine risers may require even larger-diameter buoyancy modules, which tend to increase drag and increase the tendency for VIV. For example, drilling risers of the prior art used in water depths up to about 8,000 feet generally use syntactic foam buoyancy modules which are about 48½ inches in diameter, while drilling risers of the prior art used in depths greater than 8,000 feet may use syntactic foam buoyancy modules which are about 54 to 55 inches in diameter.
Substantial effort has been expended in the prior art on means of reducing drag and/or suppressing VIV in marine risers, including but not limited-to temporarily-installed devices such as helical strakes, flags, wake splitters, and rotating generally foil-shaped fairings. In addition, permanent alterations to the surface of the marine riser buoyancy modules have been proposed, including grooves and dimples, and alterations to the surface roughness of the floatation. Further, on larger-scale bluff bodies such as SPAR structures, integral fluid passages with control valves have been proposed to suppress VIV.
For example, riser fairings of the prior art include the following. U.S. Pat. No. 4,474,129, issued to Watkins, teaches a rotatable riser pipe fairing of syntactic foam. U.S. Pat. No. 5,421,413, issued to Allen, teaches a flexible fairing, or “shroud,” surrounding a riser to reduce VIV. These shrouds may generally be denser than water, so that they would preferably be connected to the riser at the top of the shroud. U.S. Pat. No. 5,738,034, issued to Wolff, teaches streamlined fairing sections that can be installed on a drilling riser to reduce VIV. U.S. Pat. No. 6,179,524, issued to Allen, teaches a staggered fairing system for suppressing VIV of a substantially cylindrical maritime element. U.S. Pat. No. 6,223,672, issued to Allen, teaches an ultra-short fairing for suppressing VIV in substantially cylindrical marine elements. U.S. Pat. No. 7,070,361, issued to McMillen, teaches a VIV suppression fairing.
Further, International Patent Application PCT/US2008/006648, by Bernitsas, teaches adding roughness to the surface of a bluff body to modify the flow around the body and suppress VIV. Surface roughness may be defined as an “excrescence” whose thickness is no more than about 5% of the diameter of the bluff body. U.S. patent application Ser. No. 12/156,960, by McMiles, teaches a plurality of dimples, indentations or protrusions about the circumference of a bluff body to reduce drag and suppress VIV.
Still further, International Patent Application PCT/GB02/02318, by Gibson, (the '318 application) teaches a plurality of axial or helical grooves in pipe cladding, between 1 cm and 30 cm deep, to cause disruption of flow to reduce VIV. FIG. 1A shows pipe cladding according to one embodiment taught in the '318 application, wherein pipe cladding 1 has outer surface 1A, and a plurality of helical grooves 2. FIG. 1B shows “fragmentary” cross-sectional views of four different groove profiles according to the teachings of the '318 application, including “part-circular” groove 3, “oblong” groove 4, “tapered flat-bottom” groove 5, and “V-shaped” groove 6. Note that the grooves taught in FIG. 1B have substantially unradiused outer edges, and that the edges of the grooves are greater than, or substantially equal to, 90 degrees to outer surface 1A of the pipe cladding.
Strakes are disclosed in the prior art including U.S. Pat. No. 6,347,911, issued to Blair, which teaches wrap panels comprising radial rib strakes. U.S. Pat. No. 6,644,894, issued to Shu, (“the '894 patent”) teaches the use of fluid passageways to reduce and/or control VIV and drag in SPARs. These and other means in the prior art of suppressing VIV in marine risers suffer from one or more serious limitations.
First, foil-shaped riser fairings, which are generally considered the most effective prior art means of suppressing VIV, are very expensive, typically costing on the order of $1 million dollars per deep-water riser string. Further, riser fairings typically must be affixed to, and removed from, the riser as it is run and retrieved; this can add significantly to the running time of the riser, and may be dangerous, as these operations typically take place on a platform suspended over open water in the moonpool of a drilling vessel. Second, other prior art means of suppressing VIV such as helical strakes or flags, are not as effective as riser fairings, but may add significantly to the current drag on the riser. Third, solutions which are integral to the riser buoyancy, such as surface treatments, have not been shown to significantly reduce drag or effectively suppress VIV as effectively as other solutions such as riser fairings. Finally, prior art means of discrete fluid diversion (as through pipes) to suppress VIV in large diameter bluff bodies like SPARs, such as the '894 patent, have not been shown to be an effective or efficient method to suppress VIV in relatively small diameter bluff bodies such as marine risers.
Further, many marine riser buoyancy modules of the prior art, including the common clamshell-style syntactic foam buoyancy modules, are unwieldy and generally difficult to install, remove and repair. For example, syntactic foam buoyancy elements (that is, each clamshell half of the common buoyancy module) are typically about 14 feet long and weigh between 1200 and 2000 pounds each in air, depending on their intended installation depth.
Accordingly, there exists a need for buoyancy modules for marine risers which may intrinsically reduce current drag, suppress or eliminate VIV, be relatively efficient in buoyancy, and be easy to install, remove, and repair.